Subset switching means in apparatus for a key telephone system for enabling intercom substations to access trunk lines

ABSTRACT

In an attachment apparatus for a key telephone system of the type which permits a plurality of intercom substations to access a trunk telephone line via a common communication circuit, a novel subset operable switching circuit is disclosed that permits each subset access to said common communication circuit only on an individual one-at-a-time basis. Transistor circuitry is provided to lock out other stations when more than one subset tries to gain access at the same time.

Reference is made to my prior filed application for patent Ser. No.587,361, filed June 16, 1975, and now U.S. Pat. No. 4,016,372, grantedApr. 5, 1977, of which this application is a division.

BACKGROUND OF THE INVENTION

This invention relates to a key telephone apparatus and, moreparticularly, to an attachment for allowing an intercom station in a keytelephone system to obtain access to a trunk telephone line.

Key telephone systems serve to interconnect telephone substationslocated on a private premises with the telephone lines or trunk lines,as variously termed, supplied to the premises by the local telephonecompany, as well as to provide an "on-premises" communication system.Typically the key telephone system includes some substations that mayaccess the trunk lines directly and also includes a plurality ofsubstations, which are denoted "intercom substations", for providingcommunications between locations on the premises. In common keytelephone systems, one or more telephone intercom substations areconnected via two intercom lines to two common intercom circuits in thekey service unit so that any party desiring may pick up the handset atan intercom telephone substation, thereby going "off-hook" and place thesubstation on the intercom line circuit. In the event that the oneintercom circuit is in use, the party would intercept any conversationcurrently in progress and cannot use that intercom line. The party canthen position a selector "push key" provided on the telephone instrumentto the position of the second intercom line circuit and access thesecond intercom line. The user can then use the intercom line to reachanother intercom substation. Each intercom substation includes a sounderwhich may be selectively actuated over a control line either by theoperator controlling the key service unit, or by another intercomsubstation if the party at such substation dials the digits of theintercom substation wherein the key service unit decodes the dialeddigit and applies the control signal to the sounder of the calledsubstation.

Customarily the intercom stations do not have access to the trunktelephone lines. Hence while persons having access to the intercomsubstation are permitted to place a call from one intercom station toanother to carry on an on-premises conversation, such persons cannot usethe intercom substations for placing calls off the premises. Likewisecalls coming in over the trunk lines cannot be connected to the intercomsubstation. Thus when an incoming outside call is received over anincoming trunk line at the operator's key service unit, the operatorascertains the identity of the person whom the calling party desires toreach. If that person is located at an intercom substation, the operatordials the extension number of that intercom substation, and the keyservice unit of the system operates its switching devices to select aline relay associated with the units and 10 digits of the calledintercom substation. When the line relay is actuated an electricalcircuit is closed over the control line to the sounder contained in thecalled intercom substation. The called party in answer picks up thehandset and receive the information from the operator and if the calledparty desires to speak directly to the calling party, the called partymust call back over a regular substation. The called party cannot reachthe calling party over the intercom line. These key telephone systemsare of a conventional and well known structure. Typical of key telephonesystems in commercial use are the 1A22model manufactured by ITT Company,and the Litkey key telephone system distributed by Litton BTS, and theT.I.E. Company key telephone system. Conceivably there are circumstanceswhere it is desirable to allow a telephone intercom substation todirectly access a trunk line in order to avoid the aforedescribedinconvenience and to obtain a greater flexibility for an existing keytelephone system.

In my prior application for patent, Ser. No. 787,361, filed June 16,1975, of which this application is a division, a unique apparatus for akey telephone system for enabling intercom stations to access trunklines is described.

Briefly the intercom to trunk access unit of that invention includes atrunk selector which may selectively connect to a plurality of trunktelephone lines entering the key telephone system, a selector advanceunit connected to the trunk selector for advancing the selector inresponse to input pulses, such as dial pulses. A busy detector isincluded in the trunk selector and an advance-on-busy unit, which has aninput connected to the busy detector and has an output connected to theselector advance unit, to supply a pulse to the selector advance unit inresponse to an output from the busy detector whereby the trunk selectoris automatically advanced to the next trunk line. As a further aspect abusy-defeat unit for preventing selector advance-on-busy is connected toand controlled jointly by the key service unit and by the intercomsubstation to couple an output to the advance-on-busy unit. Relay means,located at the key service unit remote from the telephone substation, iscoupled to the intercom substation for coupling the intercom station tothe trunk access unit. And push button switch means are located at theintercom substation for actuating the relay means.

In the operation of that unit, the removal of the substation handsetplaces the intercom substation in the "off-hook" condition. Operation ofthe push button switch at the intercom station actuates the relay meansto couple the intercom station through the the trunk access unit. Thetrunk access unit awaits and responds to a dialed digit, such as "1",which actuates the selector advance and that in turn steps the trunkselector to the corresponding first trunk position. If the trunk is notin a busy condition the selector connects the intercom communicationline through to the first trunk line. However if the first selected lineis in a busy condition, the busy detector signals the advance-on-busycircuit which in turn supplies a control signal or pulse to the selectoradvance unit and in response the advance steps the trunk selector to thenext trunk and the next trunk is in turn tested for "busy". This processof "trunk hunting" continues until the first available non-busy trunkline is reached, whereupon the intercom line is connected through tothat trunk line.

Further in accordance with that invention, if the intercom station isplaced in an off-hook condition in response to a signal from theoperator, the busy-defeat unit receives one input, such as from the linerelay associated with the called substation. By remaining off-hook, theparty simply operates the push button switch at the intercom station andaccesses the intercom-to-trunk access unit. In so doing, the push buttonswitch further supplies the second input to the busy-defeat circuit,which thereupon disables the busy-advance unit. The party at thesubstation dials the digit of the selected trunk. The digit pulses arereceived by the selector advance unit which steps the trunk selectorfrom the initial position over to the position of the selected trunkline and the selector switches through the intercom station to the trunkline. Inasmuch as the party is responding to a call on the selectedtrunk line, the trunk line would otherwise show busy and prevent theselector from stopping at that trunk hence the busy-defeat circuitdisables the advance-on-busy unit.

It is recognized that the aforedescribed apparatus permits use of asingle communications circuit by each of the individual telephonesubsets. To ensure privacy of any call established between an intercomsubstation and an outside trunk line over that communications circuitand to ensure proper operation of the circuits limiting the access tothe aforedescribed line to only one intercom subset at any time isdesired.

SUMMARY OF THE INVENTION

A principal object of the present invention therefore is to provide inthe aforedescribed apparatus a novel circuit for allowing line access orseizure of the intercom to trunk line access unit as well as to preventmisoperation thereof. A second object of this invention is to ensureprivacy of communications carried over the communications channelthereby established.

Briefly, in accordance with this invention, in an apparatus for enablinga plurality of telephone subsets having access to a commoncommunications circuit, each of which subsets contains a hookswitch anda momentarily operable pushbutton switch means and at least two subsetcircuits, a first one of said subset circuits is completed through thehookswitch and the second one of such subset circuits is completedthrough the hookswitch and the pushbutton switch means in series. Aplurality of relays and associated relay driving transistor meanscorresponding to said plurality of pushbutton switch means is provided;each of said relay means includes a contact means for coupling saidfirst subset circuit in circuit with said common line responsive to saidassociated relay means being placed in the operated condition, saiddriving transistor means being coupled to an associated one of saidsecond subset circuits responsive to operation of the associatedpushbutton switch means during the presence of an enabling bias at thebase of such transistor means for providing operating current to saidassociated relay means; gating means associated with each pushbuttonswitch means in said second subset circuits and said relay drivertransistor means normally providing an enabling bias to the base of allsaid driving transistor means and responsive to concurrent operation ofmore than one of said pushbutton switch means for biasing all of saiddriving transistor means into the noncurrent conducting condition duringsuch concurrent operation to prevent operation of more than one of saidplurality of relay means simultaneously; a plurality of holding circuitmeans, each including said first subset circuits and a contact means ofan associated relay means for holding the associated relay means in theoperated condition responsive to operation of said relay means andirrespective of the current conducting condition of the associateddriving transistor means but only so long as said subset is in theoff-hook condition; and an inhibit circuit means including a contact oneach of said relay means coupled to all of said driving transistor meansand responsive to operation of any one of said relay means for biasingall said driving transistor means in the noncurrent conducting conditionto prevent operation of any other relay means during continued operationof any one of said relay means; and each of said relay means includingmake contact means for connecting said first subset circuit to saidcommon communications circuit.

The foregoing objects and advantages of the invention together with thestructure characteristic of the invention hereinbefore brieflysummarized are better understood by considering the detailed descriptionof the preferred embodiments of the invention which follow, consideredtogether with the figures of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 symbolically illustrates a preferred embodiment of a systemcontaining my invention; and

FIGS. 2a and 2b illustrate schematically a specific preferred embodimentof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention is illustrated in FIG. 1, partiallyschematically and partially symbolically, so as to bring out clearly andconcisely the novel relationships between apparatus characteristic ofthe invention, unencumbered with details of known apparatus notnecessary to an understanding of the invention. A key service unit ofthe conventional key telephone system is indicated symbolically by thedash lines 10. A remote intercom substation associated with the keyservice unit is indicated generally by the dash lines 20, which outlinea familiary telephone set, and an "intercom-to-trunk" access unit isindicated generally by dash lines 40. As is conventional, the keyservice unit is connected to the outside trunk telephone lines,illustrated symbolically by the nine trunk lines 11, entering the keyservice unit. As is conventional each trunk line includes three separateelectrical leads, designated ring, tip, and sleeve, and the trunk linesprovides the communications channels through the utility or "outside"telephone system to the key telephone system.

As is conventional in the key telephone systems, a plurality oftelephone lines extend from the key service unit to remote telephonesubstations, which are not illustrated, which lines are representedgenerally by the two lines, 12. And in addition a plurality of"intercom" substation telephone lines extend from the key service unitto a plurality of remotely located intercom substations. These intercomlines are represented in the figure by the three intercom lines 1, 2 and3. Only one intercom line per station is illustrated in this embodiment.In present key systems, the telephone substations connected to the keyservice unit over lines 12 through conventional circuits in the keyservice unit have and obtain direct access to the trunk lines 11.However the intercom lines, such as 1, 2 and 3, cannot access thosetrunk lines in a conventional key telephone system.

Cable 4 couples the insulated electric wires or lines of the intercomcommunication line, as hereinafter explained in greater detail, betweenintercom telephone substation 20, selected by the user to have access totrunk lines 11, and key service unit 10. Cable 4 includes a signal line5, the S or sleeve line 6, the T or tip line 7, and the R or ring line8, which are conventional. Cable 4 also includes a control line 9, whichis added in accordance with the invention.

Substation 20 includes an electrical sounder 21, a ground connection 22to electrical ground or common, a hookswitch, not illustrated, whichincludes a plurality of contacts, one of which is illustrated ashookswitch contact 23, shown in its normal position. Other structuralaspects of telephone substation 20, including the rotary type dialpulsing mechanism, the receiver and transmitter circuits, which elementsare connected through leads 7 and 8, are not illustrated inasmuch asthey are of conventional design and do not contribute to anunderstanding of the invention.

One terminal of sounder 21 is connected to electrical ground potentialvia line 22. Ground potential is connected also to the transfer contactof the normally open hookswitch contacts 23.

A spring return push button switch 24 is installed in substation 20. Thepush button switch includes a set of normally open contacts 24a. One ofthe switch contacts 24a is connected via electrical lead 25 to S line 6and the other one of switch contacts 24a is connected via electricallead 26 to control line 9.

If the telephone employed is an ITT Type 510, the telephone contains a"push-key" which includes both a push button switch and an intercom lineselector switch (or key) in one integral unit. In such telephone theequivalent push button section is used and a separate push button switchneed not be installed.

As is conventional, the key telephone system contains an intercomstation selecting apparatus for activating the signal buzzer, such assounder 21, of a selected station. This is accomplished withconventional apparatus, not illustrated, that selects the T and U leads,representative of the tens and units digits of the station selectionrelay corresponding to the tens and units digit of the selected intercomstation. One such selection relay is partially illustrated as relay 13and this relay is associated with intercom station 20. The relayincludes a set of make contacts 13-1 for completing a circuit to sounder21. An additional set of make contacts, 13-2, is included on theselection relay. One of the contacts is connected to ground potentialand the other is connected in circuit by lead 47 to an access unit 40.

An electromagnetic type relay 14, a conventional switching device ofknown structure, is provided at the location of the key service unit.The relay includes a winding 14, a first transfer contact set 17, asecond transfer contact set 18, and a third transfer contact set 19. Asrepresented in the figure, each transfer contact set includes a makecontact, a break contact, and a transfer contact which pivots frommechanical and electrical contact with the "break" contact to suchcontact with the "make" contact when the relay is energized. One end ofrelay winding 14 is connected in series with a diode 15 to control line9 and the other end of winding is connected to the negative polarityterminal of a DC voltage source -V. Diode 9 is poled so as to permitcurrent flow in a direction from "ground" to the source -V. Suitablyanother diode 16 is connected across the relay coil and poled oppositeto diode 15.

Signal lead 5 extends through to cable 3 into the key service unit,where it is connected generally to the contacts of a station selectionrelay, symbolically illustrated as 13, associated with intercom station20. The signal lead 5 is also connected via line 5a through a cable,symbolically represented as 41, to the intercom-to-trunk access unit 40.

The make contacts of each of transfer contacts 17 and 18 are connectedvia lines 8a and 7a of cable 41 into the trunk access unit. The makecontact of contact set 19 is connected by a lead to the ground side ofrelay winding 14 to establish a holding circuit. Lead 6 is alsoconnected via lead 6a to trunk access unit 40. The break contact oftransfer contact sets 17, 18 and 19, are connected to lines 8, 7 and 6,respectively, which extend into the key service unit. It is seen thatthe relay switching device in accordance with the teachings of thisinvention permits breaking the electrical circuit established over theR, T, S and SIG leads between the key service unit and the remoteintercom substation.

The intercom-to-trunk access unit 40, as is symbolically illustrated inthe figure, is a separate attachment that is coupled in circuit betweenthe trunk lines 11, the relay 11, the relay switch means 14, and calbe 4to remote intercom telephone substation 20. The functional purpose ofthe access unit as becomes apparent is to connect the intercomsubstation to an available (non-busy) trunk line or, alternatively, to aselected trunk line. Various types of known electromchanical orelectronic equipment and devices are available for together performingthe function of intercom-to-trunk access unit 40 in the mannerdescribed, including one novel circuit means illustrated in FIG. 2 andhereinafter to be described is preferred.

Those skilled in the art are familiar with the conventional step-by-stepor Strowger type trunk-hunting selector switch which when actuatedsequentially moves a set of contacts from trunk telephone line to trunkline stopping to test for a busy line condition until the firstavailable non-busy trunk line is located and a through connection fortelephone communication is thereupon made. Likewise incorporated in suchtypes of devices are means for defeating an advance-on-busy so that inresponse to signals from the telephone substration, such as dialimpulses, the selector switch can step its contacts to the selectedtrunk line, even though such trunk line is "busy", and establish aconnection therewith. These apparatus are illustrated in figuresymbolically by function within the dash lines 40.

Thus a trunk selector 41 is provided which is shown to have a connectionmovable to any one of the nine positions associated with the nine trunklines from cable 11 for connecting the telephone lines from cable 41thereto. A busy detector 42 is provided as an element to test the trunklines for a busy conditon. The busy detector has an input to the trunkselector and an output connected to a "pulse on busy" unit 45. Anadvance means 43 is provided for providing a signal to advance the trunkselector from one position to the next. The advance means has an inputconnected to the outputs of each of a "dial pulse" unit 44 and the"pulse on busy" unit 45. The dial pulse unit receives dial signalinformation, such as dial pulses, over the lines in cable 41 and signalsadvance unit 43 with the appropriate information. The pulse-on-busy unit45 is structured to supply a signal to activate the advance means whenthe busy detector detects a busy condition.

A busy-defeat unit 46 has an output connected to the busy detector 42or, as by choice, to the pulse-on-busy circuit. There are two inputs tothe busy-defeat circuit 46; one of the inputs is supplied from intercomsubstation lines via cable 41 and the other input is supplied by the keyservice units via an electrical lead 47. Both inputs must be energizedto activate busy-defeat unit 45.

Conventional power supplies necessary to operate the equipment areincluded but are not illustrated. Typically the positive terminal of thesource is connected to ground and the negative terminal forms the activelead, -V, as symbolically illustrated.

Considering a first mode of operation of the invention in which theparty at the intercom substation desires to access one of the trunklines 11, the procedure proceeds as follows: By lifting the substationhandset, station 20 goes off-hook and hookswitch contacts 23 close.Electrical ground potential via lead 22 and closed contacts 23 is placedon line 6 and applied over cable 4 into the key service unit torepresent the substation as "busy". The calling party then depressesmomentarily pushbutton switch or "access" switch 24. Switch 24 applies apath from electrical ground from line 6, lead 25, switch contacts 24a,lead 26, line 9 of cable 4, diode 15, relay coil 14 to the negativevoltage source -V. Current flows in that circuit and energizes relay 14.

Relay 14 energizes and operates its transfer contacts 17, 18 and 19 fromthe break to the make position. This opens the line 6 into the keyservice unit and closes a circuit to hold relay 14 energized, via groundover line 6 at contact set 19; and transfer contacts of 17 and 18transfers leads 7 and 8, respectively, from an electrical connectionwith the corresponding lines of key service unit 10 to an electricalconnection with the corresponding leads 7a and 8a of cable 41 to accessunit 40. The party then may operate his dial mechanism, not illustrated,by dialing the number 1 for example. The dial mechanism sends a singlesignal voltage or pulse over lines 7 and 8, and leads 7a and 8a to thedial pulse detector. The dial pulse detector 44 signals advance unit 43which in turn provides an output to the trunk selector 41 causing theselector to move from the normal "off trunk" position illustrated to aconnection with the first trunk line and the first trunk line isthereupon tested for a busy condition by busy detector 42. If the firsttrunk line tests busy, busy-detector 42 provides a signal to the"pulse-on-busy" unit 45 which in turn supplies another pulse to theadvance unit and the latter in turn causes the selector to movesequentially to the next trunk line. If the next trunk line also testsbusy, the foregoing operation on detection of a busy condition isrepeated. When the selector finally reaches a non-busy trunk, thecommunication line from the intercom substation from the input of accessunit 40 are coupled through to the selected trunk line at the output.

Once the trunk line is obtained the calling party receives dial tone andmay place an off-premises call via the trunk line, in a conventionalmanner. Upon completion of the call the party at the intercom substationreplaces the handset in its cradle which opens hookswitch contact 23,which in turn opens the holding circuit to relay 14, which restores, andall the circuits are restored to normal.

Considering now a second mode of operation: When a call is received overa trunk line at key service unit 10 and the calling party relates to theoperator a desire to reach a party who is located at remote intercomsubstation 20. The operator places the calling party on "hold" and dialsthe number of the intercom station. Switching circuitry in the keyservice unit selects the station relay, here relay 13, and applies avoltage across its T and U terminals. Relay 13 operates and applies aground over contact 13-2 to lead 47 and applies a potential via contact13-1 to signal line 5 to complete a path for current over line 5,sounder 21, lead 22 to ground. The sounder operates and signals theparty at the intercom station.

When the called party answers by lifting the handset at substation 20from its cradle, hookswitch contact 23 operates and places a ground online 6. By conventional circuits, not illustrated, this causes the keyservice unit to remove the voltage from line 5 to terminate operation ofsounder 21. The operator advises the called party of the incoming callon one of the trunk lines, for example, trunk line "5", and the calledparty while retaining his telephone off-hook, simply depressesmomentarily push button 24. Thereupon as in the aforedescribedoperation, ground potential over line 6 is placed onto control line 9resulting in the operation of relay 14 in the same manner ashereinbefore described. The second pair of contacts on relay 13, 13-2,applies a signal, such as ground potential, over line 47 to one input ofthe busy-defeat circuit 46. Likewise a signal is supplied over line 41to the second input of the busy-defeat circuit, and the busy-defeatcircuit is actuated. This is necessary since incoming trunk line 5 whichis on "hold" will test busy.

The called party then dials the digit 5. This produces a serial group offive pulses which pass over lines 7 and 8 and 7a and 8a to access unit40. In the access unit the dial pulse unit 44 relays these pulses to theadvance unit 43 which causes the selector unit to advance to the trunkline 5. Thereupon selector 41 completes the communication circuitbetween the intercom station and trunk line 5. Upon completion of thecall the called party restores the handset to its cradle in thesubstation 20, the hookswitch contact 23 opens, relay 14 restores tonormal, and the circuits are restored to their normal position.

As is also apparent, intercom station 20 operates in the system in aconventional manner as long as switch 24 is not operated. Thus telephonecalls to and from substation 20 and other intercom stations within thekey telephone system are established and proceed in a conventionalmanner.

As is apparent from the foregoing description, modifications to the keytelephone system required to incorporate the added functions of theinvention includes the incorporation of a signaling circuit at theselected intercom station, such as the switch 24; the addition of a newcable 4 between the intercom substation and the location of the accessunit; a cable from the key service unit to the access unit; the additionof a relay 14 and switching contacts, which is simply installed bybreaking the direct connection at the key service unit and wiring theleads to the relay contacts, as well as adding a make contact to theselection relay at the key service unit and extending an electricalconnection therefrom to the trunk access unit; and extending theconnection to the trunk lines to the trunk access unit.

Thus the invention allows expansion of an ordinary key telephone systemin a simple and uncomplicated manner.

A preferred embodiment of the aforedescribed invention is presentedschematically in FIGS. 2a and 2b. This embodiment is of greatercomplexity inasmuch as it includes additional features such as thecapability of handling three telephone intercom substations andemploying two intercom telephone lines as well as various additionalfeatures which hereinafter becomes more apparent to the reader. Theembodiment is illustrated using various known symbols to represent theconventional elements from which the invention is formed.

A remote intercom telephone substation 50 is symbolically represented bya rectangle. The components thereof, relevant to an understanding of theinstant invention, include a sounder 51, shunted by a protective diode53, ground connection GND, hookswitch contacts 56, push button 57 andits normally open switch contacts 57a. One of the contacts of hookswitch56 and one terminal of an electronic sounder 51 are connected in commonto the ground terminal GND. The other contact of the hookswitch isconnected to terminal S(1) in common with one of the push buttoncontacts 57a. The other contact of switch 57a is connected to terminalCO(1). The remaining terminal of sounder 51 is connected to terminalSIG(1). Terminals T₁ (1) and R₁ (1) are connected to the first intercomline similarly designated and T₂ (1) and R₂ (1) are the lines of thesecond intercom line as connected to the first remote substation. Thedesignations of T and R used herein in connection with leads andterminals refers to the tip and ring leads familiar to those skilled intelephone circuits.

The substation includes a conventional dial mechanism and transmitterand receiver apparatus, not illustrated, a selector switch to place thatapparatus in circuit with the T and R leads of either the first orsecond intercom line, as well as other conventional components that arenot here illustrated. Some telephone sets have a "push-key" switch thatcontains a selector switch for selecting one of two intercom lines and apush button switch. In those telephones the push button switch is usedas switch 57.

The terminals of telephone 50 are connected by the various illustratedelectrical leads, suitably formed into a cable, to the control circuitCCl indicated in dash lines. For convenience, the leads may be referredto hereinafter by the designation of the associated terminal. Theelements of CCl are described hereinafter in greater detail.

Similarly the conventional system key service unit is represented indash lines labeled KSU and includes extending therefrom correspondingterminals and leads T₁, R₁, T₂, R₂, S₁, S₂ and BY.

A second telephone intercom substation is represented by the rectangle60 and a third telephone intercom substation by the rectangle 70. Eachof the stations 60 and 70 are of the same structure as that of theaforedescribed station 50. Thus station 60 includes terminals GND,SIG(2), CO(2), S(2), T₁ (2), R₁ (2), T₂ (2), and R₂ (2), andcorresponding electrical leads extending therefrom to the controlcircuit CC2 associated therewith in the vicinity of the key serviceunit. And station 70 includes corresponding terminals GND, SIG(3),CO(3), S(3), T₁ (3), T₂ (3) with leads extending to the associatedcontrol circuit CC3 in the vicinity of the key service unit.

Briefly, the elements found within control circuit CC1 havecorresponding elements connected together in the same manner forperforming the same functions in each of the other control circuits CC2and CC3. Accordingly only the elements of CC1 need be described in anydetail.

Control circuit CC1 contains transistors Q10, Q11 and Q12, suitably ofan NPN type, and transistor Q13, suitably a PNP type. The base of theQ10 is connected to S(1) and a bias resistor R11 is connected betweenthe Q10 base and source -V. The collector of Q10 is grounded. A resistorR10 is connected between Q10 emitter and source -V. Emitter of Q10 isalso connected to transfer contact of a relay contact set T₁ -5 and tothe anode of a silicon controlled rectifier MCR 120 of the busy defeatgate circuit BDG1, later described. The transistor Q10 forms anelectronic switch that is biased to the "off" condition.

Transistor Q11 base is connected to terminal CO(1) and in series with abias resistor R12 to source -V. Transistor Q11 collector is connected toground. The emitter of Q11 is connected in series with a bias resistorR13 to source -V and is also connectd in circuit with input A of thesieze detector and the anode of a diode CR40, later described in greaterdetail. Transistor Q11 is an electronic switch which is biased to the"off" condition.

The corresponding transistors in control circuits CC2 and CC3 have theiremitters conncted to input B and anode of diode CR41 and to input C andanode of diode CR42, respectively.

Transistor Q12 collector is connected to terminal SIG(1). A diode CR10has its anode connected to the Q12 collector and its cathode to ground.Transistor Q12 emitter is connected to source -V₂. The base of Q12 isconnected in series with resistor R14 to the collector of transistorQ13. Transistor Q12 forms an electronic switch that is normally biasedin the "off" condition.

Transistor Q13 base is connected in series with bias resistor R16 toground and in series with a resistor R17 to the transfer contact ofcontact set TE1-2 of relay TE1, described hereinafter. Additionally adide CR12 has its anode connected to base of Q10 and its cathode to baseof Q13. Another diode CR13 has its anode grounded and its cathodeconnected to emitter of transistor Q13. Collector of Q13 is connected inseries with resistor R15 to source -V2 and to anode of diode CR11. Thecathode of diode CR11 is connected to the transfer contact of relaycontact set TE1-3 and in series with a resistor R to the gate of asilicon controlled rectifier MCR 120.

A station selection relay TE1 is provided. The relay winding isconnected in series with a diode CR14 between terminals UN1 and TN1,which although not illustrated are connected in circuit with theselection circuits of conventional structure located in the key serviceunit KSU. The corresponding station selection relays, not illustrated,in each of CC2 and CC3 are connected in circuit with terminals UN2 andTN2 and UN3 and TN3, respectively, and via those terminals to the keyservice unit.

Relay TE1 contains make contact sets TE1-1, TE1-2 and TE1-3. The fixedor make contact of TE1-2 is connected to source -V; the make contact orrelay contacts TE1-3 is connected in multiple with the correspondingrelay contacts in each of CC2 and CC3 to terminal BY of the key serviceunit KSU. Relay contact set TE1-1 has its transfer contact connected tobreak contact of relay contacts T1-5 and the make contact of TE1-1 isconnected to KSU terminals S1 and S2 in series with diodes CR5 and CR6via connection to the anodes thereof and in multiple with thecorresponding relay contact in each of CC2 and CC3.

Control relays T1, T2 and T3 are associated with a corresponding one ofcontrol circuits CC1, CC2 and CC3. Control relay T1 has six sets oftransfer contacts T1-1, T1-2, T1-3, T1-4 and T1-5, illustrated withinCC1 and T1-6.

Each of relays T2 and T3 have corresponding contacts, not illustrated,in each of CC2 and CC3, and have contact sets T2-6 and T3-6,respectively.

The transfer contacts in relay contact sets T1-1, T1-2, T1-3 and T1-4are connected to terminals T₁ (1), R₁ (1), T₂ (1), and R₂ (1) of thetelephone set, respectively. The break contacts thereof are connected toterminals T₁, R₁, T₂ and R₂, respectively, of the key service unit KSU,and in multiple to the corresponding circuit locations in each of CC2and CC3.

The first and second intercom circuits, not illustrated, in the keyservice unit KSU, are connected in circuit, respectively, with one pairof the T₁ and R₁ and T₂ and R₂ terminals.

The make contact of relay contact sets T1-1 and T1-3 are connected incommon and to the transfer contact of line relay contact set K1-1 and inmultiple to the corresponding relay contacts, not illustrated, in CC2and CC3.

The make contact of relay contact sets T1-1 and T1-4 are connected incommon and to transfer contact of line relay contact set K1-2 and inmultiple to corresponding relay contacts in each of CC2 and CC3.

As becomes apparent hereinafter, an intercom line T and R lead is placedin circuit with the corresponding tip (T) and ring (R) leads to anoutside trunk line by means of the relay contacts K1-1 and K1-2, ashereinafter described.

A detecting circuit SD, which I term a "seize detector", is provided toallow only one remote station to access the unit at any given period oftime. Hence, if more than one remote station attempts to gain access tothe unit, as described in greater detail hereinafter in connection withthe description of operation, the drive transistors Q6, Q7 and Q8 areforward biased only if one unit attempts to access at one time. Theseize detector consists of four conventional logic AND gates designatedSD-1, SD-2, SD-3 and SD-4, symbolically illustrated. AND gates SD-1,SD-2, and SD-3 have two inputs each labeled a₁, b₁, a₂, b₂, and a₃ andb₃, respectively. One input of gate SD-1, a₁ to the second b₂ input ofAND gate SD-2, the second input b₁ of SD-1 is connected to the secondinput b₃ of gate SD-3; the second input b₂ of gate SD-2 is connected tothe first input a₃ of gate SD-3. The outputs from each of the gates areconnected to a respective one of the three inputs of gate SD-4 and theoutput terminal of gate SD-4 provides the output of seize detector SD.For convenience, the three inputs to SD are labeled A, B an C and inputA is connected to the emitter of transistor Q11 and inputs B and C areconnected to the emitter of the corresponding transistor in CC2 and CC3.

Transistors Q6, Q7 and Q8, suitably of the PNP type, are provided asdrivers for relays T₁, T₂ and T₃, respectively. The emitter of Q6 isconnected to the cathode of a diode CR40; the emitter of Q7 is connectedto the cathode of a diode CR41; the emitter of Q8 is connected to thecathode of a diode CR42; the collector of Q6 is connected in seriescircuit with the winding of relay T1 to source -V; the collector of Q7in series with relay T2 winding to source -V; and the collector of Q8 inseries with relay winding T3 to source -V. Each of the three relaywindings is shunted by a diode, CR43, CR44 and CR45, poled so as not tonormally conduct current, respectively, as transient voltage protectionfor the associated transistor.

The make contact of relay contact set T1-5 in control circuit CC1, isconnected to the collector of Q5 and relay T1 winding via a lead I mayterm a hold output lead; a correspnding lead from CC2 is connected tocollector of Q7; and a corresponding lead from CC3 is connected tocollector of Q8 so as to provide a current path to the associated relaywinding in a "hold" circuit.

The output of seize detector gate SD4 is connected in series withresistor R40 to the base of each of transistors Q6, Q7 and Q8.

Busy Defeat Detector

Four conventional logic AND gates, represented by schematic symbols,form what I term a busy defeat detector, represented within the dashlines formed into a rectangle labeled BDD. This includes AND gates BD1,BD2 and BD3, which are of the two input type, and gate BD4 which is ofthe three input type. The construction, including power sourceconnections, of these conventional logic gate devices are well known andneed not be considered in detail. The respective first and second inputsof each of gates BD1 through BD3 form the six inputs of the busy defeatdetector as shown. The outputs of each gate BD1, BD2, and BD3 isconnected to a corresponding input of AND gate BD4. The output of thegate BD4 is connected to the base of a transistor Q5. Transistor Q5,suitably an NPN type, additionally has its collector connected to groundand its emitter connected in multiple to each of the corresponding 133 .. . 933 inputs of the select circuits SC1 . . . SC9 illustrated in FIG.2b, hereinafter described in greater detail, and functions as a "busydefeat driver" electronic switch.

A device I term a "busy defeat gate" is provided for each of the controlcircuits and is represented in dash lines as BDG-1, BDG-2 and BDG-3 withthe details illustrated only for BDG-1. This novel circuit includes asilicon controlled rectifier, abbreviated SCR, MCR 120, resistor R18 andresistor R19. The anode of MCR 120 is connected to the emitter oftransistor Q10 located in control CC1. The cathode end of MCR 120 isconnected in series with resistor R18 to source -V and is connected tothe second input of AND gate BD1 in the busy defeat detector BDD. Thegate of the SCR is connected in series with resistor R to the transfercontact of relay contact set TE 1-3 located as illustrated in controlcircuit CC1. Resistor R19 is connected between the SCR's gate andcathode.

Similarly, the corresponding components in each of BDG-2 an BDG-3 areconnected to the second inputs of AND gates BD2 and BD3 and tocorresponding elements in each of control circuits CC2 and CC3.

Hold Detector

Six logic AND gates of conventional structure, represented byconventional schematic symbols, form what I term a "hold detector"outlined in dash lines HD. This includes AND gates HD1, HD2, HD3, HD5and HD6 which are of the two-input type, and AND gate HD4 which is athree-input type. The output of each of AND gates HD1, HD2 and HD3 areconnected to a corresponding one of the three inputs of AND gate HD4.The output of AND gate HD4 is connected to one input of gate HD6 inseries with resistor R41. A capacitor, C40, is connected between thatinput of HD6 and source -V. One input of AND gate HD5 is connected tomake contact of relay contact set K9-3 and in series circuit with biasresistor R5 to source -V. The second input of gate HD5 is connected toelectrical ground. The output of gate HD5 is connected to the remainingone of the two inputs of AND gate HD6. That remaining input of gate HD6is also connected in circuit with terminal E of electronic counter 101,illustrated in FIG. 2b, hereinafter later described.

The output of gate HD6 is connected to the base of a switchingtransistor Q4. Transistor Q4, suitably an NPN type, has its collectorconnected to ground and its emitter connected to the transfer contact ofrelay contacts K1-3. Additionally a resistor R43 is connected betweenthe emitter and ground and a second resistor R42 is connected betweenthe emitter and source -V for biasing the transistor.

Input 1 of gate HD1, input 1 of gate BD1, make contact or relay contactsT1-6, the anode of diode CR48, and one end of resistor R47 are connectedin common by the illustrated electrical lead.

Input 1 of gate HD2, input 1 of gate BD2, make contact of relay contactsT2-6, anode of diode CR47 and one end of resistor R46 are connected incommon by the illustrated electrical lead.

Input 1 of gate HD3, input 1 of gate BD2, make contact of relay contactsT3-6, anode of diode CR48 and one end of resistor R45 are connected incommon by the illustrated electrical lead.

The cathode ends of diodes CR46, CR47 and CR48 are connected in commonto the base of each of the driver transistors Q6, Q7 and Q8. Theremaining end of bias resistors R45, R46 and R47, substantiallyidentical to one another, is connected to source -V.

The input 2 to each of AND gates HD1, HD2, and HD3 is connected to thecorresponding one of the anode ends of diodes CR40, CR41 and CR42,respectively.

Each trunk telephone line includes a corresponding T-lead, R-lead andH-lead. The respective T, R and H leads of trunk line "1", notillustrated, are connected electrically to terminals T1, R1 and H1; therespective T, R and H leads of trunk lines 2, not illustrated, areconnected respectively to terminals T2, R2 and H2, and the T, R and Hleads of trunk line 9 are connected to the respective terminals T9, R9and H9. Likewise, similar terminals and connections for the T, R and Hleads of trunk lines "3" through "8" are not illustrated but arerepresented in FIG. 2 by the dash lines in between trunk line 2 andtrunk line 9 in the figure.

Each of the other trunk lines likewise has associated therewith a relay,not illustrated, as may be designated relays K2 through K9. Anelectromechanical type relay K1, shown on the right hand side of theFIG. 2b in circuit SC1, is associated with trunk line 1. Relay K1includes three transfer contact sets K1-1, K1-2 and K1-3 associated withthe corresponding leads for trunk line 1 on the left side of the figure.A similar three transfer contact sets K9-1, K9-2 and K9-3 are shown fora relay designated K9, not illustrated, and sets K2-1, K2-2 and K2-3 fora relay designated K2, not illustrated. Each of the other relays K3through K9 unillustrated likewise contain three transfer contact sets.

Make contact of set K1-1 is connected to terminal T1; make contact ofset K1-2 is connected to terminal R1; and make contact of set K1-3 isconnected to terminal H1. These terminals are associated with circuitryfor trunk line 1, not illustrated. In like manner, as illustrated, themake contacts of contact sets K2-1, K2-2 and K2-3 are connected toterminals T2, R2 and H2, respectively, associated with trunk line 2, andthe make contact of sets K9-1, K9-2 and K9-3 are connected to theterminals T9, R9 and H9, respectively, associated with trunk line 9.Included, but not illustrated, the make contacts of each of the transfercontacts set associated with the described relays K3, K4, K5, K6, K7,K8, which are not illustrated, are connected to corresponding R, T and Hterminals, not illustrated, associated with a corresponding one of theother trunk lines 3, 4, 5, 6, 7 and 8.

As illustrated, the break contact of the first set of relay K-1 isconnected to the transfer contact of the corresponding contact set onthe second relay, and so on up to the eighth relay which has its breakcontact of the first set connected to the transfer contact of the firstcontact set K9-1 of relay K9, not illustrated. As illustrated, a likeelectrical connection is made between the break contact of K1-2 throughto the transfer contact of set K9-2, and from the break contact of K1-3through to the transfer contact of K9-3. The set of relay contacts andtrunk relays establishes a circuit for interconnecting the R ring and Ttip leads, to which remote intercom substation 50 is connected, and ahold H lead to corresponding leads of one of the nine trunk lines,depending upon which one of line relays K1 through K9 of the selectcircuits SC1 through SC9 is energized in the manner subsequentlydescribed.

As is shown, normally a circuit exists between the tip or T lead, fromtelephone 50, 60 and 70, the transfer contact and break contact oftransfer set K1-1 through similar transfer contacts and break contactsof each of the other relay contact transfer sets K2 through K8 and thetransfer contact and break contact of contact set K9-1 through diode CR1into the base of a transistor Q2. By a similar circuit, the ring or Rlead from the remote intercom telephone stations normally is connectedthrough the transfer contacts and break contacts K1-2 through K9-2 tocircuit ground. A circuit for the hold driver is completed through thethird transfer contact sets K1-3 and K2-3 through K9-3 to the emitter oftransistor Q4.

Transistor Q2, suitably a PNP type, contains an emitter, base, andcollector. The base of transistor Q2 is coupled in series with diode CR1to the break contact of contacts K9-1. Diode CR1 is poled to allowcurrent to pass in a direction from the relay break contact of K9-1 tothe transistor base. A bias resistor R4 is connected between the base ofQ2 and the source -V. A capacitor C2 is connected between the base andemitter of Q2 and output resistor R3 is connected between source -V andthe Q2 collector. A diode CR3 has its cathode connected to the emitterof Q2 and its anode connected to ground. The circuit is the structure ofa conventional pulse converter circuit which at its input receivesvoltage pulses of varying widths and shapes and provides a standardpulse as its output.

A diode CR7, poled as illustrated to conduct current, and a resistor R2are connected in series between the collector of transistor Q2 and thebase of another transistor Q1, suitably an NPN type transistor. Acapacitor C1 is connected between the cathode of diode CR7 and source-V. A resistor R1 is connected between the base and the source -V andthe emitter of transistor Q1 is similarly connected to source -V. Aresistor 100K is connected between Q1, collector and ground. The circuitstructure of Q1 is a conventional structure, sometimes referred to as adelay circuit, in which response to a pulse at its input switches intothe On condition and remains in that condition for a predeterminedperiod of time and thereupon restores to its Off condition if nosubsequent pulses are received within that predetermined time period andswitches into its current conducting condition until a predeterminedperiod after the last pulse to its base input. This I designate as an"interdigit storage circuit".

A resistor R8 is connected between the collector of transistor Q2 andthe input of pulse shaper circuit PS, the latter of which is ofconventional structure. The pulse shaper comprises a conventionalsemiconductor NAND gate connected as an inverter having three inputterminals with one input for receiving the input signal and with theremaining two inputs connected to electrical ground potential. Theoutput of pulse shaper PS is connected to one input of OR gate OR1 inFIG. 2b.

Element 101 represented in FIG. 2b is a semiconductor decimal counter ofconventional structure, such as the type 4017A counter available fromthe Motorola or RCA Companies as type MC14017CP and CD4017CP,respectively. Briefly, such a counter, when in the reset condition,produces a high on the "0" position output and provides in sequence a"high" level output on the 1 to 9 outputs designated in the figure, forevery "high" going pulse applied at its input T and, on receipt of thetenth pulse, again produces a high at the "0" output or reset condition.The counter includes a reset terminal designated R which is connected bya bias resistor 103 to source -B and to the break contact of relaytransfer contact set T3-6 in FIG. 2a. Ground is connected to thetransfer contact of T1-6 and the break contact thereof is connected tothe transfer contact of T2-6. Similarly the break contact thereof isconnected to the transfer contact of the contact set T3-6. The Eterminal of counter 101 is connected to one input of gate HD-6 of thehold detector and the T input is connected to the output of OR gate OR1.

A certain portion of the apparatus and circuits are schematicallyillustrated within a rectangle formed by dash lines SC1 which I denoteas a select circuit. Such apparatus and circuitry, as is illustratedwithin the dash lines SC1, is identical to such as is included, but notillustrated, within the rectangles formed by dash lines SC2, SC3, SC4,SC5, SC6, SC7, SC8 and SC9. Each of these circuits is associated with anindividual one of the nine trunk lines designated TR1 through TR9 asshown, and extend through key service unit KSU by leads, notillustrated.

Select circuit SC1 includes a three-input NOR gate 121 of conventionalstructure as is illustrated by the NOR symbol. A NOR gate is a familiarlogic circuit element which provides an output of a "low" if any of itsinputs has a "high" applied to it. The first input thereof is connectedin series with a diode CR101, poled as illustrated, to an input terminalSC1a, and is also connected in series with a bias resistor R101 tosource -V. The second input is connected in series with a diode CR102 toa terminal SC1b and in series with a bias resistor R102 to source -V;and the third input is connected in series with a diode CR103 to aninput terminal SC1c and is also connected in series with a bias resistorR103 to source -V.

By means of electrical leads, only partially illustrated, terminal SC1ais connected to the incoming call indication circuit of the first trunkline TR1; terminal SC1b is connected electrically to the answer circuitof the first trunk line TR1 and terminal SC1c is connected to the holdcircuit of TR1, which circuits are found within the prior art keytelephone system. As hereinafter explained in greater detail, inoperation the NOR gate functions as a busy detector which provides anoutput if trunk line 1 is busy.

Transistor Q101 is suitably an NPN type transistor and functions as anelectronic switch. The output of NOR gate 121 is connected to the baseof transistor Q101 in series with a resistor R104. The emitter of Q101is connected to the source of potential -V and the collector thereof isconnected to the emitter of another transistor Q102 and is furtherconnected in series with a resistor R106 to electrical ground potentialand to an output reference point 135. A charging capacitor C101 isconnected between the base of Q101 and the source -V.

An additional circuit extends from the input 133 in series with a diodeCR104, poled as illustrated, and large resistance R105 to the base oftransistor Q101.

Transistor Q102 is suitably an NPN type and functions as an electronicswitch. This transistor is considered in conjunction with anotherswitching transistor Q103, also an NPN type.

Transistor Q102 has a diode CR105 connected in series between the inputat 134 and the base of Q102, with polarity as illustrated. A biasresistor R107 is connected between the base of Q102 and electricalground potential. Transistor Q103 has its base connected in circuit witha circuit input 136, has its collector connected to electrical groundpotential, and has its emitter connected to one end of the relay windingof relay K1. The other end of relay winding of K1 is connected to thetransfer contact of the transfer contact set K1-4 of relay K1. Thecollector of aforementioned transistor Q102 is also connected to thetransfer contact of relay contact set K1-4. This forms an electricalseries circuit between the collector of Q102 and the emitter of Q103.

The make contact of contact set K1-4 is connected to the source -V toprovide a hold circuit for the relay. Additionally, the winding of relayK1 is shunted by a diode CR106, poled as illustrated so as not tonormally conduct current.

A like circuit with the same components for each of the remaining 8trunk lines is included in this embodiment and this is represented bythe figure simply by the dashed rectangles labeled SC2, SC3, SC4, SC5,SC6, SC7, SC8 and SC9, and the associated input and output terminals.

However for completeness of this description, major circuit elementswhich correspond in function and circuit to those in diagram SC1 areidentified in the vertical columns as follows, which should assist in anunderstanding of the mode of operation hereafter considered:

    ______________________________________                                        CIR-  NOR     TRAN-    COMPO-  RE-  CON-  TRAN-                               CUIT  GATE    SISTOR   NENT    LAY  TACT  SISTOR                              ______________________________________                                        SC1   121     Q101     Q102    K1   K1-4  Q103                                SC2   221     Q201     Q202    K2   K2-4  Q203                                SC3   321     Q301     Q302    K3   K3-4  Q303                                SC4   421     Q401     Q402    K4   K4-4  Q403                                SC5   521     Q501     Q502    K5   K5-4  Q503                                SC6   621     Q601     Q602    K6   K6-4  Q603                                SC7   721     Q701     Q702    K7   K7-4  Q703                                SC8   821     Q801     Q802    K8   K8-4  Q803                                SC9   921     Q901     Q902    K9   K9-4  Q903                                ______________________________________                                    

And the inputs and output of the other circuits and their correspondenceto the inputs and outputs of the specifically illustrated circuit SC1are set forth in the following columns:

    ______________________________________                                        CIRCUIT    INPUT AND OUTPUTS                                                  ______________________________________                                        SC1        133       134       135    136                                     SC2        233       234       235    236                                     SC3        333       334       335    336                                     SC4        433       434       435    436                                     SC5        533       534       535    536                                     SC6        633       634       635    636                                     SC7        733       734       735    736                                     SC8        833       834       835    836                                     SC9        933       934       N.A.   936                                     ______________________________________                                    

The emitter of transistor Q5, previously described in connection withthe busy defeat detector, has its emitter connected electrically incommon with all of the inputs 133, 233, 333, 433, 533, 633, 733, 833 and933 of the select circuits as illustrated.

The collector of transistor Q1, in addition to its connection to input134, is connected in common to all of the other corresponding circuitinputs 234, 334, 434, 534, 634, 734, 834 and 934 of the select circuit.

A series of eight NAND gates of conventional structure N1, N2, N3, N4,N5, N6, N7 and N8, as designated and symbolically illustrated in FIG.2b, are provided corresponding in number to one less than the number ofselect circuits SC1- SC9. Each of those NAND gates includes three inputsand a single output as is illustrated.

A first input of each of the NAND gates N1 through N8 is connected bylead 139 to the collector of transistor Q1.

The second input of gate N1 is connected to output terminal 135 ofcircuit SC1 and thereby is placed in circuit with the collector oftransistor Q101. The second input of each of the remaining gates, N2through N8, is connected to a corresponding terminal output, 235, 335,435, 535, 735 and 835 in each of the select circuits SC1 through SC9,respectively.

The third input of gate N1 is connected to the input 136, placing it incircuit with the base of transistor Q103. A like connection is made froma corresponding location in the circuit of each of the remainingcircuits, SC2 through SC8, namely inputs 236, 336, 436, 536, 636, 736and 836 to the third input of NAND gates N2 through N8, respectively.

The 1, 2, 3, 4, 5, 6, 7, 8 and 9 outputs of electronic counter 101 areconnected to corresponding ones of the inputs 136, 236, 336, 436, 536,636, 736, 836 and 936, of select circuits SC1-SC9, respectively.

Two additional NAND gates N9 and N10, connected as an "OR" gate, and anOR gate OR2 are provided as represented by conventional graphicalsymbols. Gates N9 and N10 each have four inputs and one output and gateOR2 is a two-input single output device. As is illustrated, the outputsof gates N1 through N4 are connected in series with a corresponding oneof resistors RN1, RN2, RN3 and RN4 to a corresponding one of the fourinputs of NAND gate N9. Likewise, each of the outputs of NAND gates N5through N8 are connected to an individual one of the four inputs of NANDgate N10 in series with a corresponding one of the four resistors RN5,RN6, RN7 and RN8. The output of AND gate N10 is connected in circuitwith one input of OR gate OR2 and the output of NAND gate N9 isconnected to the remaining input of that OR gate. In turn, the output ofOR gate OR2 is connected to one of the two inputs of NOR gate OR1.

It is noted that the inherent input capacitance of the NAND gatetogether with the appropriate input resistor forms a time delay circuithaving a slight time delay between the appearance of an output at a oneof the gates N1 through N8 and the presentation of a sufficient voltageto an input of one of the NAND gates N9 and N10 and this time delay isessential in the operation of this circuit as becomes apparent from thedescription of operation of the invention explained hereinafter.

A dial and busy tone generator is included and symbolically illustratedas DBT. This is an integrated circuit chip, such as the Signetics Model556 available from the Signetics Company, Sunnyvale, California. Thecircuit configuration and ancillary components and power supplies arenot illustrated inasmuch as they do not form a part of the invention andare otherwise available in the literature of the Signetics Company.Briefly, the device generates a steady 400 hertz electrical signal,representative of a line seizure signal, unless a voltage is applied atits control input, in which case the 400 hertz signal is interrupted ata one-half hertz rate, representative of a busy signal. The output ofthe device is designated DBT-1 and a control input DBT-2 areillustrated. Input DBT-2 is connected to output "0" of electroniccounter 101 and output DBT-1 is connected in series with a capacitor andresistor, not illustrated, to the break contact of relay contact setK9-1.

The power source is of any conventional type capable of supplying thedesired DC voltages of -V and --V₂. This is represented by the rectanglelabeled PS in the lower left portion of FIG. 2a and by its outputslabeled -V and -V2.

The operation of this embodiment is next described.

When the remote user at station 50 lifts his handset to place either anordinary intercom call or to place an outside trunk call, groundpotential, a voltage high, is applied from the closed hookswitch contact56, terminal S(1), to the base of transistor Q10 and via diode CR12 tothe base of transistor Q13. With ground so applied, transistor Q13 isbiased to the Off condition so that its collector goes low. This low iscoupled to the base of transistor Q12 via resistor R14 to, in turn, biastransistor Q12 to the Off or noncurrent-conducting condition. Withtransistor Q12 in the Off condition, current cannot flow through sounder51 and the sounder cannot be operated. Additionally with a voltage lowat the collector of Q13, acknowledgement over the BY lead in the keyservice unit is prevented since diode CR11 in that instance blocks acurrent path should relay contacts TE1-3 be closed for any reason.

With the telephone handset off-hook, a standard telephone intercom pathis established through contacts T1-1 and T1-2 or contacts T1-3 and T1-4to the corresponding T1 and R1 and T2 and R2 leads of the key serviceunit KSU to accomplish the normal functions of a remote intercom unit.If the remote user now wishes to place a call over the trunk lines, heoperates push button 57. In so doing, ground potential is applied viahookswitch 56, push button contacts 57a, terminal CO(1) to the base oftransistor Q11. That biases transistor Q11 to the On condition andprovides a voltage high at both one end of diode CR40 and to one inputof gates SD1 and SD2 of seize detector SD.

Transistor Q6, Q7 and Q8 are the relay drivers associated with thetransfer relays T1, T2 and T3, respectively, which are in turnassociated with corresponding ones of the three intercom stations 50, 60and 70. Transistors Q6, Q7 and Q8 are normally electrically biased inthe On condition by a voltage applied to the respective transistor basesin common from the output of the seize detector gate SD4 via resistorR40. This output voltage is normally a voltage low. The transistorscannot switch to "on" until a source of voltage is connected to theemitter. With transistor Q6 biased to the On condition and with avoltage high applied to its emitter, current flows through thetransistor and the winding of relay T1. Relay T1 is energized andoperates its contacts T1-1, T1-2, T1-3, T1-4, T1-5 and T1-6. Thistransfers the lines T1-1, R1-1, T2-1 and R2-1 from connection to the keyservice unit to connection with the tip and ring leads to the transfercontacts K1-1 and K1-2 of relay K1.

At this juncture in this description of the operation of the invention,the operation of the device I term the seize detector is considered. Theseize detector prevents two stations from seizing the line-to-trunkaccess unit simultaneously since the unit can only service one suchstation at a time. As described previously, in switching "on",transistor Q11 of control circuit CC1 places a voltage high at one inputof each of gates SD1 and SD2 via SD input A. If either of the remotestations 60 or 70 attempts to access the trunk to line unit concurrentlywith station 50, another voltage high appears at another input of one ofthe NAND gates SD1, SD2 or SD3. For example, if station 60 concurrentlyattempts to access the trunk unit, a voltage high is provided at input Bof the associated control circuit which results in a high being placedat an input of gate SD3 and at the other input of gate SD1. In thatevent, both inputs to gate SD1 are at high and the output of that gateswitches from high to low. With the gate output low applied to the inputof gate SD4, gate SD4 then switches its output from low to high and thusremoves the "low" bias from the base of each of the transistors Q6, Q7and Q8. Those transistors then cannot be switched to the "on" condition.With transistors Q6, Q7 and Q8 being held in the Off condition, all ofthe corresponding transfer relays T1, T2 or T3 are prevented fromoperating and further operation is thus prevented until one of theremote station users releases his push button, such as 57 at station 50,and the output of the seize detectors again becomes a voltage high.

Assuming either the latter case or that the remote station 50 is thesole station seeking access, relay T1 operates, and a unique dial toneis returned to the station over the trunk to line access unit.

As was explained in connection with the description of the constructionof the preferred embodiment, the telephone handset and related equipmentare not illustrated in the FIGS. 2a and 2b since they are not necessaryto an understanding of the invention. Likewise the switch which allowsthe local substation to switch between lines T1, R1, and lines T2, R2,is not disclosed. For convenience, we assume that the switch isconnected to lines T1 and R1.

The dial and busy tone generator, DBT, properly connected to sources ofpower, not illustrated, generates and supplies at output DBT-1 asuitable continuous signal such as 400 hertz which is a unique toneassociated with the line to trunk access unit. With relay T1 operatedand line to trunk access unit free, the dial tone is supplied to theuser at remote station 50 via the output BDT₁ via the lead to the breakcontact of contact set K9-1 and the transfer contact thereof and throughthe similar chain of contacts on relays K3 through K8, not illustrated,the transfer and break contacts K2-1 and the transfer and break contactsK1-1 to the tip line, and thus indicates to the user that the accessunit is free for use. The transfer contact of relay contact T1-5 swingsto its position with the make contact. The operation of hookswitch 56 tothe closed position placed ground at the base of transistor Q10 and,accordingly, transistor Q10 is in the On condition so that its emitteris at a voltage high. This high is applied via the transfer and makecontacts of contact set T1-5 to one side of the relay winding T1, thuscurrent through transistor Q1, contacts T1-5, the relay winding of T1 tothe source -V maintains the relay T1 operated and the contacts T1-5 forma relay holding circuit. Thus even though the driver transistor Q6 isthereafter switched to its Off condition, the transfer relay T1 remainsoperated.

Relay contact T1-6, shown on the right hand side of FIG. 2a, swings fromthe position in contact with the break contact to the make contact tocomplete a circuit from voltage high, namely ground potential, throughto the diode CR48, and there to the base of each of the transistors Q6,Q7 and Q8 to bias them into the Off condition. This voltage overridesany voltage applied from the seize detector, thereby preventing anyother attempted seizure.

Additionally the ground is supplied to input 1 of gate BD1 in the busydefeat detector BDD and to input 1 of gate HD1 in the hold detector HDfor purposes hereinafter discussed. Additionally the ground is removedfrom the circuit comprising the transfer and break contacts of each ofthe relays T1-6, T2-6 and T3-6 to input R of the electronic counter 101in FIG. 2b. This enables the electronic counter.

Transistor Q2 functions as a pulse detector and is normally biased viaresistor R4 in the On condition. Once the line to trunk access unit hasbeen seized by station 50 ground, a voltage high, is applied to the baseof Q2 via the following circuit: ground, the break and transfer contactsK9-2, through the contact chain and through the break and transfercontacts K1-2, through the R1 lead and transfer contact T1-2, throughthe closed circuit in the telephone instrument, not illustrated,including the dial pulsing contacts, out over line T1-1, transfercontact T1-1 and make contact, line T1, over to the transfer contactsK1-1 and the break contact thereof, through the relay chain, includingtransfer and break contacts K9-1, through diode CR1 to the base oftransistor Q2. Transistor Q2, accordingly, is placed in the Offcondition and its collector is at a negative or voltage low. Thisnegative voltage is applied via resistor R8 to one input of pulse shaperPS.

Transistor Q1 is normally in the On condition with a voltage highapplied to its base, such as occurs in the case of transistor Q2 beingin the current conducting condition. With transistor Q2 now in the Offcondition, the voltage high is removed from the anode of diode CR7 andthe negative bias from source -V through resistor R1 provides thedominant bias at the base of transistor Q1 and, accordingly, transistorQ1 switches into its Off condition and its collector, accordingly, goesfrom a low to a voltage high condition. The output of the collector oftransistor Q1 is applied to one of the inputs of each of the gates N1through N8 in the scan detector, and is applied as well to each of theinputs 134, 234, 334, 434, 534, 634, 734, 834 and 934 in selectorcircuits SC1 through SC9 which are illustrated in FIG. 2b. Consideringthe select circuit SC1 as typical, the high at input 133 biasestransistor Q102 to be in the On condition once other circuits areestablished as hereinafter becomes apparent.

The key service unit, KSU, circuits provide three inputs, SC1a, SC1b,SC1c, to the three inputs of NoR gate 121; thus if any of the ringing,answer or hold circuits in the key service unit associated with trunkline 1 are operating, a voltage high is applied to at least one of therespective inputs. With a high at any one of the three inputs, theoutput of busy detector 121 is low. However if the trunk line is free,all inputs to the NOR gate 121 are low causing the output of 121 to behigh. This high is applied to the base of transistor Q101 via resistorR104.

Transistor Q101 is normally in the Off condition, however with a highapplied to its base the transistor Q101 is placed in the On conditionmaking its collector go to a voltage low. Inasmuch as the base oftransistor Q102 is already at a voltage high, transistor Q102 is readyto conduct current if a load appears in the collector circuit. As isshown, the load in the collector circuit of Q102 is the winding of relayK1. Thus, if a proper voltage appears at input 136 of select circuitSC1, transistor Q103 operates to conduct current through relay K1 andthrough the collector to emitter circuit of Q102, causing transistor K1to operate.

Assume now that the digit 1 is dialed. As soon as the dial breaks forthe first pulse the ground is removed from the base of transistor Q2.With ground removed, transistor Q2 is biased to the On condition andconducts current causing its collector to go to a voltage high.Collector high forward biases the diode CR7, charges the capacitor C1,and biases transistor Q1 to its On condition. Transistor Q1 conductscurrent and causes its collector to go to a voltage low. This changesthe bias on transistors Q102 and the equivalent transistors in each ofthe remaining select circuits SC2 through SC9, causing them to be biasedinto the Off condition.

The capacitor C1 is of such a value that it stays charged during thepulse period as occurs during dialing. For example, if the number 5 isdialed there will be five pulses separated by a between or interpulseperiod. In the case of the number 1 there is only one pulse, so themajor time delay effect of capacitor C1 is not important. During theperiod in which capacitor C1 has not discharged sufficiently, transistorQ1 in each of the circuits remains "on" during dialing, and each of thetransistors Q102 and the corresponding transistors in the remainingselect circuits SC2 through SC9 remain in the Off condition. Thisprevents any of the relays K1, illustrated, and K2-K9, not illustrated,associated with SC1-SC9 from operating until dialing is completed. Atthe same time the collector of transistor Q2 being high causes theoutput of pulse shaper PS to go low and that, in turn, changes the stateof the clock driver, gate OR1, and the output of that clock driver goeshigh. The high is applied to the trigger or T input of electroniccounter 101 in FIG. 2b. Electronic counter 101 is a decimal counter ofconventional structure which counts the high going dial pulses and givesa high output in sequence on its 1 to 9 and "0" outputs corresponding tothe number of pulses received at its input. The unit is reset to zerowhen a high, such as ground, appears on the R input, and when reset tozero a high appears at its zero output. It is noted that the zero outputis coupled to input BDT-2 of the dial and busy tone generator circuitfor purposes hereinafter explained.

Pulse shaper PS, ideally a NAND inverter gate, is biased via resistor R8to the voltage at the collector of transistor Q2 to normally have avoltage high on its output. This high is applied to one input of OR gateOR1, the clock drive unit, and causes the clock driver to normally haveits output at a low. This low is applied to the trigger input T of theelectronic counter.

As previously described, relay T1, contacts T1-6, removed ground fromthe R input of the counter. Accordingly, the electronic counter is readyto receive dialing pulses.

In a rotary dial system, dialing produces an interruption of theelectrical circuit to the telephone for the duration of the pulse. Aspreviously described, upon seizure of the line to trunk access unit acircuit was completed from ground through the telephone set to the baseof transistor Q2. Accordingly, when the dial pulse contacts interruptthe circuit the transistor Q2 follows these interruptions and switchesbetween its On and Off conditions. Thus, for example, for the number 5the dial pulse contacts interrupt the high applied to the base oftransistor Q2 five times in sequence. For the dialing of the number 1this interruption occurs but once. Thus when the dial interrupts theline to generate the dial pulse, ground is removed from the base oftransistor Q2 and causes transistor Q2 to turn on. Accordingly Q2 causesits collector to go to a voltage high. This high at the collector of Q2biases the diode, charges capacitor C1 and turns on transistor Q1.

Transistor Q1 turns on and makes its collector go to the low condition,turning off transistors Q102 and the equivalent transistors in each ofthe circuits SC2 through SC9.

At the conclusion of the dial pulse a high appears on the 1 output ofthe electronic counter. As is shown the 1 output of the counter iscoupled to input 135 of select circuit SC1, and from there to the baseof relay driver transistor Q103. Additionally this high is applied to aninput of gate N1 in the scan detector for reasons which later becomeapparent. With a high at the base of Q103 the transistor is switchedinto its On or current conducting condition.

Returning now to the consideraton of the transistor Q1, after asufficient period of discharge of capacitor C1 governed by the values ofthe resistances R2 and R1 and the absence of additional dial pulses,transistor Q1 switches back into its Off condition, placing a voltagehigh at input 133. And accordingly, transistor Q102 is again biased tothe On condition. With both transistor Q102 and Q103 biased to the Oncondition, a current path is completed through ground resistor R106, theemitter and collector of Q102, the winding of relay K1, the emitter andcollector of Q103, and energizes relay K1. Relay K1 operates and closesits contacts K1-4 which places one end of the relay winding K1 at thepotential of source -V to complete a holding circuit for the relay. Thusrelay K1 remains operated as long as transistor K103 is biased in the Oncondition.

As previously explained, each of the relays corresponding to K1 areassociated with a corresponding one of the trunk lines. Accordingly,when relay K1 operates, it operates its contacts K1-1, K1-2 and K1-3seen in FIG. 2a. This completes a circuit between the leads of station50 and the T1, R1 and H1 leads associated with the first trunk line inthe line card of the conventional key service unit. At that time theparty at station 50 can further dial over the trunk line to complete anoutside call.

Although the foregoing example was given in connection with the dialingof the number 1, it is seen that the same result occurs were the stationuser to dial the number 5. However, in that instance, transistor Q2would have switched on and off five times in sequence. The output ofpulse shaper PS would have similarly switched on and off five separatetimes, and the output of clock driver OR1 would have provided fivedistinct pulses to the input of counter 101, and a high would haveappeared at the 5 output of the counter.

During the period of occurrence of this series of pulses, transistor Q1would have switched off upon the first dial interruption and would haveremained off during the period between the dial pulses, thus preventingany of the relays in the select circuits SC1-SC9 from inadvertentlyoperating.

In the next example of the operation of the invention, one considers thecircuit operation which occurs in the event that the selected trunk lineis busy. As becomes apparent, the output of OR gate OR2 suppliesautomatically an additional pulse to OR gate OR1 and that gate in turnsupplies a pulse to the counter 101 to advance the counter to the nextposition.

Thus if trunk line 1 is busy there will be at least one high at one ofthe inputs of busy detector 121. Accordingly, the output of gate 121 islow. This "low" in turn biases busy driver transistor Q101 to the Offcondition. With transistor Q101 off, the collector is at a voltage high,as appears through resistor 106 to ground. And the output 135, which isconnected to an input of gate N1, is at a voltage high. Thus with trunkline 1 busy at the conclusion of the dialed digit "1", the output oftransistor Q1 goes "high" when transistor Q1 switches off, and via lead139 applies a high to the first input of gate N1. The output of SC1 atterminal 135 is also at a voltage high in accordance with the previousassumptions, which is applied to the second input of gate N1. And withthe electronic counter 101 stepped to the first position the 1 output isat a voltage high and this is applied to the third input of N1.Accordingly, all three inputs to gate N1 are at a voltage high causingthe output thereof to go low. With a low at its input NAND gate N9output switches from a low to a high and this is supplied to the inputof OR gate OR2. In turn, OR gate OR2 switches its output condition froma high to a low and applies that low to the 1 input of OR gate OR1, andthe OR gate, in turn, applies an additional high pulse to the triggerinput T of the electronic counter. It is noted that the resistor RN1operates in conjunction with the input capacitance of the NAND gate toprovide a slight time delay to ensure that any additional pulse arrivesat the trigger input of counter 101 subsequent to the time when anyearlier input is terminated. The same useful delay effect occurs witheach of the other RN resistors and the associated NAND gates so that anypulses that are generated in this way are separated or spaced in timeand do not overlap, in which latter case the counter would not bestepped. Accordingly, the output of the counter advances and supplies ahigh at output 2. Assuming now that the second select circuit SC2 issimilarly busy, the output at 235 will be high, which again, as in thepreceding case, makes all three inputs to the gate N2 at a high, andaccordingly another pulse is generated through the action of gates N2,N9, the OR gate OR2, gate OR1, trigger input of the counter, and thecounter advances to the next position resulting in a high at the 3output. Assuming the trunk line associated with SC3 is not busy, thenthe output at 334 of SC3 is low and the gate N3 will not switch itsstate. Subsequently, the corresponding relay in circuit SC3,corresponding to relay K1 in SC1, and its associated driver relay willoperate to close a line relay K3, not illustrated, associated therewithand connect the appropriate lines from station 50 to the trunk line viathe relay contacts K3-1, K3-2 and K3-3 in the relay chain in FIG. 2acorresponding to the illustrated relay contacts K1-1, K1-2 and K1-3 inFIG. 2a.

All Trunks Busy

In the event that all of the nine trunk lines associated with thecircuit are busy, counter 101 steps its output in the aforedescribedmanner to the ninth position and stops. The low at the "0" position inturn is applied to input DBT-2 of the dial and busy tone generator. TheDBT circuit detects the low and internally causes the 400 hertz signalto be interrupted at the rate of one-half hertz so as to generate adistinctive busy tone signal. This signal is applied over the sameoutput DBT-2 and over the lines including the K9-1 through K1-1contacts, snce none of these relays operated, to the user's telephone atsubstation 50 in the same transmission path that was used for thepassing of the original access tone.

Receiving an Incoming Call

Since the line to trunk access unit detects any trunk that is answeredor on hold as busy, the selector circuit will by-pass that trunk if itis dialed by the user in the manner previously described, unless furtheraction is taken. In this regard, a busy detector defeat circuit isprovided. When an incoming trunk call appears at the key service unit,the operator at the key service unit may access the party at the remoteintercom station, such as station 50, by dialing the number associatedwith that remote station, and through conventional circuitry in the keyservice unit this applies an operating current through relay TE1 anddiode CR14 connected between terminals UN1 and TN1 to operate thisrelay. Relay TE1 closes its contacts TE1, TE2 and TE3. Contact TE1-2places a voltage low at the base of transistor Q13 if remote 50 is not"off-hook". Q13 switches on, causing its collector to go high, which inturn biases transistors Q12 to the On condition. Transistor Q12completes a circuit from ground through the sounder 51 and the collectorto emitter circuit thereof to pass current through the sounder 51 so asto alert the party at station 50. Additionally the high at the collectorof Q13 is applied to the gate electrode of silicon controlled rectifierMCR120 to prepare MCR120 to conduct current.

It is noted that if station 50 is off-hook, the voltage high (ground)applied via diode CR12 to the base of transistor Q13 overrides thevoltage low applied via resistor R17 to maintain transistor Q17 in the"off" condition and thereby prevent sounder 51 from being energized atthis time.

When the party at the remote station removes the handset to answer thecall, the hookswitch 56 closes completing a ground to the base oftransistor Q10, and Q10 is biased thereby to its On condition, placing ahigh at its emitter. This high is applied to the anode of MCR120. Thuswith the anode of MCR120 at a high, the cathode connected via resistorR18 to a low and the gate at a high, the SCR switches "on". Thisswitches the input at 2 of gate BD1 in the busy defeat detector BDD tobecome high.

After the operator at the key service unit relates the information tothe user at station 50 and the user desires to access the trunk line onwhich the incoming call awaits, the user must then obtain the accessunit by depressing push button 57. Those closes circuit ground a high tothe base of transistor Q11 as in the previous case, causing transistorQ11 to turn on. This seizes and holds the transfer relay associated withstation 50, T1, in the same manner as described in the previousdescription of operaion. However relay contacts T1-6 transfers ground, avoltage high, to the make contact in the set and this is coupled overthe illustrated leads to the first input of gate BD1 in the busy defeatdetector. Accordingly, a high is presented at both inputs of AND gateBD1 and the output of AND gate BD1 goes "low". This low is applied tothe one input of the busy defeat detector AND gate BD4. Normally busydefeat detector gate BD4 had a high at each of its three inputs and,accordingly, its output was low. Accordingly, with one input at a lowthe output of the gate BD4 goes high to bias the base of busy defeatdriver transistor Q5.

With a high at its base, transistor Q5 is biased into the On condition.This completes a circuit through the transistor to ground through eachof the inputs 131 through 931 in the select circuits. Taking the circuitSC1 as typical, with input 131 at a voltage high the base of busy drivertransistor Q101 is biased to the On condition regardless of thecondition of any of the associated busy detectors, such as busy detector121 in circuit SC1, and a "low" is provided to output 135 of SC1.Accordingly the user may then dial the appropriate trunk line. Givingtrunk line 5 as an example, the user dials 5 and five pulses, as occursin the preceding description, steps the electronic counter 101 to itsfifth position resulting in a high at output 5. Inasmuch as the outputsof each of the select circuits SC1 through 9, namely 535, is at avoltage low, the scan detector, gates N1 . . . N8, does not operate toperform the automatic trunk hunting feature. As in the precedingdescription, the line relay associated with the line corresonding torelay K5, not illustrated, in SC5 operates and holds. In so doing, theappropriate set of relay contacts in the chain illustrated in FIG. 2aoperates to connect the user at station 50 to the appropriate trunkline.

Hold (Litkey-Tie)

Once the user is connected to the appropriate trunk line, a circuit"ground" necessary to place the line on hold is furnished by transistorQ4, the hold driver, which is normally on.

Normally, gate HD-1, HD-2 and HD-3 of the hold detector HD have a low ontheir input and, in turn, the output of each is at a high. These highsare connected to gate HD-4 input normally making the output of HD-4 avoltage low. The low output of HD-4 is connected on one input of gateHD-6.

The output of gate HD-5 is low until a trunk line is seized. With lowson the two inputs of gate HD-6, its output is at a high and this high isapplied to the base of transistor Q4, biasing Q4 in the "on" condition.

When a trunk line is seized, the high applied to the input of HD-5 viathe K1-3 . . . to K9-3 contacts is interrupted and the input goes low.Hence the output of HD-5 goes high, which high, in turn, is coupled toone input of HD-6. However, HD-6 does not provide a low at its outputuntil a high is present on each of its inputs.

When station 50 seizes the line to trunk access unit relay, contact T1-6places a high on one input of HD-1. If the user wishes to place the callon hold, he depresses push button 57. This action turns on transistorQ11 and places a high on the remaining input of gate HD-1. The outputthereof goes low and causes the output of gate HD-4 to go high. Thereare now two highs on the inputs of HD-6, and the output of gate HD-6goes low, biasing off transistor Q4. With Q4 off, resistor R43 nowfurnishes the ground to the H-lead of the trunk line. While maintainingthe push button 57 depressed, the user replaces his handset on itscradle and the station thereby goes "on-hook", removing ground potentialfrom the H-lead. This sequence is recognized by equipment in the keyservice unit KSU, which places the trunk line in the "hold" condition.

The foregoing description of the preferred embodiment of my combinationand subcombination inventions is sufficient in detail to enable oneskilled in the art to practice and use the invention. However it isexpressely understood that my invention is not to be limited to thosedetails presented for that purpose inasmuch as many modifications andsubstitutions of equivalent elements, all of which are within the spiritof my invention, become apparent to those skilled in the art uponreading this specification. Accordingly it is respectfully requestedthat my invention be broadly construed within the full spirit and scopeof the appended claims.

What I claim is:
 1. A selection device for connecting one of a plurality of electrical input circuits to a single electrical output circuit comprising:a plurality of switch means, one associated with each of said input circuits for initiating a circuit selection; a corresponding plurality of electromagnetic relay means, each having a winding; a corresponding plurality of transistor means, each of said transistor means having a base, emitter and collector; means connecting the associated ones of said switch means, said emitter, said collector and the relay winding in an electrical series circuit across a source of voltage; gate means normally providing an output to the base of all said transistor means for biasing same to a current conducting condition and responsive to operation of more than one of said switch means concurrently for providing an output to the base of each of said plurality of transistor means to bias all said transistor means in the noncurrent conducting condition during simultaneous operation of more than one switch means; whereby operation of a single one of said switch means completes a current conducting path through the emitter of the associated transistor means and through the associated relay winding in series to energize said relay means; a plurality of holding circuit means responsive to operation of an associated one of said relay means for completing an electrical holding circuit to the associated winding thereof to maintain said relay means operated irrespective of the current conducting condition thereafter of the associated transistor means; and inhibit means responsive to any one of said relay means being in the operated condition for providing an output to the base of all said transistor means to bias said transistor means into the concurrent conducting condition; a plurality of relay contact means, one contact means associated with each relay means and with one of said circut inputs and said single electrical output, for completing an electrical circuit between a respective input and the output.
 2. The invention as defined in claim 1 wherein said inhibit means comprises a plurality of contact means, one associated with each of said relay means, for applyinga bias voltage from a source to said base of each transistor means responsive to energization of the associated relay means.
 3. In a telephone system containing a plurality of telephone subsets, an electrical output circuit and an electrical input associated with each of said telephone subsets, each of said telephone subsets contains hookswitch means operable between an on-hook and off-hook condition, and a selection means for selectively connecting any one of said electrical input circuits to said electrical output circuit, the improvement therein comprising:a plurality of switch means, one associated with each of said input circuits for initiating a circuit selection; a corresponding plurality of electromagnetic relay means, each having a winding; a corresponding plurality of transistor means, each of said transistor means having a base, emitter and collector; means connecting the associated ones of said switch means, said emitter, said collector and the relay winding in an electrical series circuit across a source of voltage; gate means normally providing an output to the base of all said transistor means for biasing same to a current conducting condition and responsive to operation of more than one of said switch means concurrently for providing an output to the base of each of said plurality of transistor means to bias all said transistor means in the noncurrent conducting condition during simultaneous operation of more than one switch means; whereby operation of a single one of said switch means completes a current conducting path through the emitter of the associated transistor means and through the associated relay winding in series to energize said relay means; a plurality of holding circuit means, one associated with each subset, responsive to operation of an associated one of said relay means for completing an electrical holding circuit to the associated winding thereof to maintain said relay means operated during said hookswitch means of the associated subset being in the off-hook condition irrespective of the current conducting condition thereafter of the associated transistor means; and inhibit means responsive to any one of said relay means being in the operated condition for providing an output to the base of all said transistor means to bias said transistor means into the noncurrent conducting condition; a plurality of relay contact means, one contact means associated with each relay means and with an associated one of said circuit inputs of an associated subset and said single electrical output, for completing an electrical circuit between a respective input and the output.
 4. The invention as defined in claim 3 wherein each of said switch means completes the said series circuit through an associated hookswitch means, said hookswitch means being in the off-hook condition. 